They found the following physiological effects after 2-4 weeks of detraining:

VO2 max: down 4-10%

Blood volume: down 5-10%

Heart rate: up 5-10%

Stroke volume: down 6-12%

Flexibility: Decreases

Lactate threshold: Decreases

Muscle glycogen levels: down 20-30%

Aerobic enzyme activity: Decreases

Running economy: Unchanged

So basically your body gets hit across the board! Of course, your mileage may vary depending on your level of fitness (the more you have, the more you have to lose), the amount of inactivity you had (bed rest vs just no training) and most likely your age (the older you are, the harder it is to regain what is lost).

This really depends on your fitness level and of course the length of the break, but in general, yes, a few weeks of illness (anything over 7-10 days in my experience) will have your endurance drop. The more training you do, the steeper the drop will be, on the other hand you will also regain it more quickly. For the average fitness person (i.e. non (semi-)professional athlete) the drop shouldn't be too bad (but noticeable, nonetheless).

Having said this, if you are sick, you shouldn't train. Rather take the drop in endurance than risk a more severe issue.

Being ill for an extended (more than a 3-4 days) will always cause your endurance to drop. However, if you've reached a plateau in strength training due to excessive workouts, an extended rest will often cause a significant jump in performance, as you've built muscle that your internal organs could not previously support.